Electric circuit breaker operating mechanism and improved supporting plates therefor



F. H. MURPHRY 3,265,833

Aug. 9,1966

ELECTRIC CIRCUIT BREAKER OPERATING MECHANISM AND IMPROVED SUPPORTINGPLATES THEREFOR Filed June 3, 1964 2 Sheets- Sheet 1 I I I I7 2115 57 3|1 FIG. 1

INVENTOR. /C;FANK H MURPHY A TTORNEY 2.Sheets-Sheet 2 INVENTOR. FRANK HMUR HY F. H. MURPHY FIGS ELECTRIC CIRCUIT BREAKER OPERATING MECHANISMAND IMPROVED SUPPORTING PLATES THEREFOR Filed June 3, 1964 FIGS UnitedStates Patent ELECTRIC CIRCUET BREAKER OPERATENG MECHANISM AND IMPROVEDSUPPORTING PLATES THEREFOR Frank H. Murphy, West Hartford, Conrn,assiguor to General ElectricCornpany, a corporation of New York FiledJune 3, 1964, Ser. No. 372,193 3 Claims. (El. 200-116) My inventionrelates to electric circuit breakers, and more particularly tomulti-pole electric circuit breakers of high current capacity typeincluding separable contacts operated by quick-make, quick-break typeoperating mechanism, which breakers are capable of interruptingshort-circuit value currents as well as intermediate value overloadcurrents.

It is a general object of the present invention to provide a multi-poleelectric circuit breaker of the molded insulating case type having anelectrical current carrying and short-circuit current interruptingrating substantially double that of comparable prior art circuitbreakers of comparable size. More particularly, it is a general "objectof the invention to provide an electric circuit breaker having a currentcarrying rating of 225 amperesat 240 volts, the external dimensions ofwhich are not ap preciably greater than the dimensions of the mostdirectly comparable prior art circuit breaker having a rating of 100amperes at 240 volts.

Two basic problems are encountered in attempting to increase theelectrical current rating of circuit protective devices of theshort-circuit interrupting type. These are (1) to increase the size ofthe current carrying elements of the device, and especially of thecontact members, so as to be able to carry the substantially increasedcurrent without'excessive heating, and (2) to provide in the limitedspace, an operating mechanism of sufiicient strength to furnishsubstantially increased contact pressure under normal conditions, and tobe able to move the larger contact members to open position fast enoughto interrupt the higher overload and short-circuit currents.

Electric circuit breakers of the multipole type ordinarily comprise aninsulating casing having a plurality of side-by-side chambers. Each suchchamber includes a stationary contact and a movable contact. Operatingmechanism is also provided for moving the movable contacts between openand closed circuit positions. In accordance with the prior art a singleoperating mechanism is usually provided, positioned in one of the polechambers, the movable contact members being ganged together on a commoncross-bar for simultaneous operation by the single operating mechanism.The size of the operating mechanism which may be used is of courselimited by the fact that the pole chamber in which the operatingmechanism is located must'also providespace for a stationary contact andassociated arc extinguishing chamber and also for a current responsivetrip device.

Various attempts have been made to provide stronger operating mechanismfor operating higher rated contacts, such attempts including (1) theprovision of a separate operating mechanism for each pole of themultipole breaker, and (2) the provision of a separate chamber.alongside the contact-containing pole chambers, such additional chamberhousing only the operating mechanism. The use of a separate mechanismfor each pole chamber, however, is extremely expensive. The use of aseparate pole chamber for housing the operating mechanism requires thatthe overall dimensions ofvthe device be increased by an amount necessaryto provide this additional space. This, of course, is contrary to theoverall objective which is to provide a breaker of increased rating tilllesser volume.

In accordance with the present invention a more powerful and effectiveoperating mechanism is provided in a pole space of comparable size ofprior art breakers by using a part of the pole chamber for two purposes,that is, to house the stationary contact and associated arcextinguisher, and also to provide support for a portion of the operatingmechanism. Thus a portion of the operating mechanism, in effect,overlaps the stationary contact area and projects over the arcextinguisher. This is true although the operating mechanism is supportedon the base portion of a top-and-bottom divided insulating housing.

In accordance with the invention in one form, an electric circuitbreaker is provided of the type including an insulating casing'having atleast one pole chamber supporting a stationary contact and an associatedarc extinguisher at one end portion of the chamber. An operatingmechanism is also provided including a movable contact member, theoperating mechanism being supported adjacent the arc extinguisher andarranged to move the movable contact into and out of engagement with thestationary contact. The operating mechanism, moreover,

includes a portion within the pole chamber extending over a portion ofthe arc extinguisher and over the stationary contact, and anotherportion supported on the same general plane as the stationary contact.

In accordance with another aspect of the invention, the operatingmechanism includes a pair of side frames, each of which includes asupporting portion engaging the base of the insulating support at apoint spaced from and generally co-planar with the stationary contactand a second supporting portion resting on the insulating base at apoint substantially directly above the stationary contact, each of thesesupporting points representing points at which the operating mechanismrests on the base. Fastening means is also provided for drawing anintermediate part of the'mechanism toward the supporting base, but notinto contact with it, to fasten the operaing mechanism in place in thecasing. The mechanismside plates therefore incorporate a bridge-typemounting, with supports at either end and fastening means intermediatethe supporting points.

In the drawings,

FIGURE 1 is an-elevational sectional view of an electric circuit breakerincorporating the invention, taken substantially along the line 11 ofFIGURE 2, the supporting side plate of the operating mechanism nearestthe viewer being omitted to show the parts better;

FIGURE 2 is a plan view of a three-pole elelctric circuit breakerincorporating the invention, a portion of the casing cover being brokenaway;

' FIGURE 3 is a side elevation view of the operating mechanism of thecircuit breaker o f FIGURE 1, the parts being shown in the offcondition;

FIGURE 4 is a perspective view of a portion of the operating mechanismof the circuit breaker of FIGURE 1, and v FIGURE 5 is a fragmentaryelevational view of portions of the circuit breaker of FIGURE 1, showingparticularly the mounting of the mechanism in the casing.

Referring to the drawings, the invention is shown in FIGURES 1 and 2 asincorporated in a three-pole electric'circuit breaker'includi ng agenerally rectangular insulating casing comprising a base 10 and a cover11. The base 10-includes opposed integral side wall portions 12'(onlyone shown) and intermediate partition portions 13 (only one shown)defining three elongated generally rectangular recesses or pole chambers14 (only two shown). The cover includes corresponding side and partitionportions, which correspond to the side and partition portions 12 and 13of the base respectively to subminal conductor or strap stantiallytotally enclose the pole chambers 14. (The side portions of the cover 11are not shown, and only one partition portion '13 is shown, see FIGURE2.) The cover I I is attached to the base by suitable means such as byscrews 11'.

Each of the pole chambers 14 has a stationary contact 15, supported on aline terminal conductor or strap 16. Each of the line terminal straps 16is fastened to the base 10 by suitable means such as by pairs of screws17, and carries a line connecting lug 18 attached thereto by suitablemeans, not shown. Each of the line lugs 18 is of the screw clamp typeand includes a clamping screw 19.

The opposite end of the base 10 includes a load ter- 20 for each polechamber, also attached to the base 10 by suitable means such as by pairsof screws 21, and support load connecting lugs 22 each includingclamping screws 23.

Each of the load straps 20 has a heater conductor 24, including aU-shaped portion passing around a magnetic field piece 25, and havingits inner end connected to a pair of flexible conductors 26 by means ofa screw 27, which also serves tofixedly attach this end of the conductor24 to the base 10.

The other end of each of the flexible conductors 26 is rigidly attachedby suitable means, such as by brazing, to movable contact members 27.The movable contact members 27 are each pivotally carried by a generallyU-shaped contact carrier 28 by means of an inverted U-shaped clamp 29and a pivot pin 30. (Note: One side of the U-shaped carrier is brokenaway.) Each of the movable contact members 27 includes a contactingsurface portion 27' disposed and arranged to engage a correspondingstationary contact 15. A compression spring 31 is disposed between thebight portion of each of the contact carriers 28 and an end portion ofthe corresponding movable contact member 27 to provide contact pressurewhen the mechanism is in the closed circuit position as indicated inFIGURE 1. A stop pin 32 is carried by the contact carrier member at theopposite side of the movable contact member 27 from the compressionspring 31, to serve as a stop to limit rotation of the contact carriermember 27 under the influence of spring 31 when the parts are in theopen circuit position as indicated for example in FIGURE 3.

The current path through each of the poles of the circuit breaker maytherefore be traced as follows: current enters at line terminal 18,flows through line terminal strap 16 to stationary contact 15, tomovable contact surface 27, to movable contact member 27, to flexibleconductor 26, to heater conductor 24, to load terminal strap 20, to loadlug 22.

, The contact carrier 28 for the central pole of the circuit breaker, asillustrated in FIGURE 1, is pivotally supported by a fixed pivot pin 33,which in turn is supported by a pair of mechanism side plates 34. Thecontact carriers 28 of each of the three poles are ganged together forsimultaneous operation by a contact crossarm 35 which is generallysquare in cross-section, and comprises a metallic core member 35A and agenerally rectangular insulating sheath 35B. Each of the contactcarriers 28 is rigidly attached to the cross-arm 35 by suitable meanssuch as by means of a generally U-shaped clamping member 36.

For the purpose of aiding in the extinction of an arc drawn between themovable contact surface 27' and the stationary contact 15, an arcextinguisher assembly is provided comprising a pair of side plates 38 ofinsulating fiber and a back wall 38' also of insulating fiber. Aplurality of spaced ferro-magnetic metallic plates 39 are supported inspaced parallel relation [between the arc chute side plates 38. Each ofthe plates 39 is generally U-shaped and the aligned openings of theplates 39 define a passageway through which the outer end portion of themovable contact member 27 moves in going from 4 closed to open circuitposition. An arc drawn between the contact members 27' and 15 istherefore drawn into the magnetic plates 39, and the arc ga-sses arevented through a screen member 40.

OPERATING MECHANISM For the purpose of moving the center contact carrier28, and with it the two outside contact carriers 28, with a quick-makeand quick-break action, an operating mechanism is provided, which willnot be described. The operating mechanism not only serves to move themovable contact members 27 between open and closed circuit positions,but also moves the contact members from closed circuit to open circuitposition upon the occurrence of predetermined overload and/orshort-circuit conditions in the circuit, and indicates such a trippedcondition by movement of the operating handle to a trip signallingposition. In addition to the side plates 34, the mounting and operationof which will be described later, the operating mechanism includes ahandle assembly comprising a handle member 41 which is supported on amanually operable member 42 of metallic material, pivotally supported ona pair of projections 43 carried by and projecting outwardly of the sideplates 34 of the mechanism. The operating mechanism also includes areleasable cam member 44, which is pivotally supported on a fixed pivotpin 45 extending between the side plates 34. The releasable cam member44 includes a curved cam surface or edge portion 46, for a purpose to bedescribed, and a latched end portion 47.

The releasable cam member 44 is normally restrained from rotation in acounterclockwise direction as viewed in FIGURE 1 by means of arestraining latch 48 carried by a common trip bar 49 extending acrossall three poles of the breaker, and pivotally supported at its oppositeends by suitable means such as by pivot pins 50 journalled incorresponding recesses in the side walls of the casing of the base 10.The common trip bar 49 is adapted to be rotated, so as to cause releaseof the latch member 48, by the action of current responsive meansassociated with any of the three poles of the circuit breaker andcomprising for each pole an elongated bimetallic strip 51 rigidlyattached to the heater conductor 24 at one end by suitable means such asby welding or brazing. Each of the bimetallic strips 51 carries anadjusting or calibrating screw 52. Each of the bimetallic strips 51 isdisposed and arranged so that when heated by the heater conductor 24, itwarps so as to move the calibrating screw 52 to the left as viewed inFIGURE 1, rotating the trip bar in counterclockwise direction to releasethe latch 48. Magnetic current responsive means is also provided foreach pole of the circuit breaker comprising a pivotally supportedarmature member 53 pivotally supported by lugs 53A extending sidewaysinto corresponding pivot recesses 54 in opposed confronting portions ofthe side walls 12 and partitions 13 of the base 10. Each of the arma-vtures 53 includes extension portions or legs 53B which are disposed andarranged to be attracted by the end portions of magnetic field piece 25upon the passage of predetermined currents through the conductor 24.Attraction of the legs 53 toward the field piece 25 causescounterclockwise rotation of the armature 53 and causes its upper end toengage the trip bar 49, rotating it in counterclockwise direction tocause tripping as previously described. A light biasing spring 55 isprovided for biasing each of the armatures 53 in clockwise direction,each of the springs 55 extending between a lug portion 53C of thecorresponding armature 53 and the lower edge portion of the magneticfield piece 25. Certain aspects of the thermal-magnetic tripconstruction disclosed herein are the subject of a separate applicationSerial No. 359,567, Iencks and Murphy, filed April 14, 1964, andassigned to the same assignee as the present invention.

The central contact carrier member 28 has a generally U-shaped operatingyoke 57 which is pivotally connected thereto by pivot pin 30 which alsopivotally supports the movable contact member 27 on thecontact carrier28, the pin 30 passing through aligned holes 57A in the yoke member 57,see FIGURE 4. The upper ends of the legs of the yoke member 57 carry acam roller supporting pin 58, which in turn supports a cam roller 59 andone end of each of a pair of tension type operating springs 60. Becauseof the use of a single integral member comprising the U-shaped member57, there is substantially no danger of the pin 58 swinging about anaxis perpendicular to its axis of rotation such as might occur if thetwo side portions 57B and 57C were separate link members.

The ca-rn'roller 59 includes a pair of opposed flange portions 59A whichfacilitate travel of the cam roller 59 along the cam surface 46 of thereleasable cam member 44. The upper ends of the tension type operatingsprings 60 are supported by the bight portion of generally U- shapedmanually operable member 42 by suitable means, such as by having a loopportion thereof extending through slots in the manually operable memberand being trapped against movement therethrough by retaining pins 61.

In operation, when the parts are in the closed circuit position as shownin FIGURE 1, the tension type operating springs, acting between thesupporting pin 58 and the bight portion of the manually operable member42, tend to cause movement of the cam roller 59 toward the left asviewed in FIGURE 1 because the center of curvature of the cam surface 46lies to the right of the line of action of the spring in this position.Movement of the cam roller 59 and its pivot pin 58 to the position shownin FIGURE 1 forces the contact carrier member in counterclockwisedirection, bringing the'movable contact surface 27' of the movablecontact member 27 into engagement with the stationary contact 15. Asthis occurs, the contact member 27 is rotated slightly clockwise aboutits pivotal support 30 on the contact carrier 28, compressing thecontact pressure spring 31.

Movement of the handle member to the right as viewed in FIGURE 1, movesthe upper ends of the springs 60 to the right, this movement continuesuntil the line of action of the springs passes to the right of thecenter of curvature of the cam surface 46. When this occurs, theoperating springs 60 move the cam roller 59 to the right, moving thecontact carrier 28 and the associated movable contact members 27clockwise about the pivotal support 33. Movement of the central contactcarrier 28 inclockwise direction is limited by engagement of the pin 30carried by the-central contact carrier 28 with edge portions of the sideplates 34 forming a corner. See particularly FIG- URE 5. This movementtakes place with a snap action, independent of the rate of movement ofthe handle 41.

When the parts are in the closed circuit position as shown in FIGURE 1,an overload or short circuit condition which causes rotation of the tripbar 49 as described above causes release of the releasable member 44from the latch 48. When this occurs, the force of the operating springs60 pulling upward on the cam roller pin 58 urging the cam roller 59against the cam surface 46, causes counterclockwise rotation of thereleasable member 44 about its pivotal support 45. This permits movementof the cam roller 59 upwardly and slightly to the right even though thehandle member 41 is held in the on position, and so permits automaticopening of the contact members. Certain aspects of the mechanismconstruction described are set forth and claimed in my copendingdivisional application Serial Number 510,113, filed October 21, 1965 andassigned to the same assignee as the present invention. These aspectsare illustrated particularly in FIGURE 4.

As illustrated in FIGURES 1 and 5, and particularly in FIGURE 5, themechanism side plates 34 each includes a first leg portion 62, whichrests against the back wall 16A of the base 10. The side plates 34 arealso supported at the opposite end by means of a cross pin 63 whichextends through the side plates 34 and has its end portions wouldexistif this end of the frame resting in notches 64 in the inner sidesof the partitions 13 of the base 10. It will be observed that the sideplates 34 merely rest against the base 10 at the support points 62 and63. The plates 34 are drawn toward the back wall 10A of the base 10 bymeans of screws 65 which thread into alternately offset portions 66 ofthe plates 34. The secrew 65 pulls the intermediate portion 67 of theside plates 34 toward the back wall 10A.

As shown in FIGURE 5, the side plates 34, and particularly the dependingportions 62 and 67, are dimensioned so that the pin 63 engages the notch64 and the leg 62 engages the back wall 10A of the casing 10 before thelower edge of the intermediate portion 67 can engage the back wall 10Aof the casing. Thus, when the screw 65 is in fully tightened conditionas shown in FIGURE 5, a small s ace 68 exists between the lower edge ofthe projection 67 and the back wall 10A of the casing base 10. Thus onlya single fastening element is used for each side frame 34. It will alsobe observed that by means of the mounting structure shown, the sideframes 34 are supported both at the extreme front and extreme rearportion, that is, at the opposite ends thereof, thereby providing a verysecure mounting, although the space used at the back wall of the casing10 for the mounting is relatively small. It will be observed that thepin 63 which rests in the notches 64 extends substantially directly overthe stationary contact 15. Thus the forward part of this pole chamber isused for two purposes, (1) to house a portion of the arc extinguisherstructure including the stationary contact, and (2) to contain andsupport a portion of the operating mechanism. Since the supporting pointof the pin 63 is at the upper edge of the base portion 10, it issufiiciently removed from the supporting point of the stationary contact15 and the stationary contact supporting strap 16 to prevent thepossibility of a dielectric oversurface failure between these two pointswhich danger were supported adjacent the back wall of the casing.

The handle support member 42 is bifurcated at its lower end to fit overthe supporting pivot pin 43. In addition, the handle member 42 includesenlarged portions 42A adjacent the lower end which serve the purpose ofretaining the pin 30 in position when the parts are assembled in thecenter pole, thereby preventing the pin 30 in this pole from slidinglengthwise to a position where it might be struck by the edge of thehandle support member 42. The corresponding pins 30 in the outside polemembers are each retained in place by the corresponding side wall 12 andadjacent partition 13.

It will be observed that the entire mechanism assembly is held inassembled relation without the need for any special retaining meansholding any of the pivot pins in their position. Thus the pins 30 and 32are retained in place, although free to slide lengthwise, by theadjacent partitions and side wall portions of the base. Also, since theside portions 57B and 57C comprise integral parts of a single U-shapedmember, no separate retaining means is required to retain these portionson the corresponding ends of the pin 58. In assembly, the pin 58 issnapped in between the side portions 57B and 57C by springing theseparts apart slightly.

The entire circuit breaker therefore has its assembly greatlyfacilitated since virtually all of the parts may be assembled by handand without any metalworking operations such as riveting, welding,brazing, etc. to hold the parts together. In accordance with theinvention, an extremely compact high spring force mechanism is provided.Thus for example, the contact pressure of the most nearly similar priorart circuit breaker of this size is about 5 pounds, whereas in theconstruction in accordance with applicants invention, the contactpressure achieved is from 8 to 10 pounds.

While the invention has been disclosed in only one particularembodiment, it will be readily appreciated that numerous modificationsthereof may readily be made. It

is therefore intended by the appended claims to cover all suchmodifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric circuit breaker comprising:

(a) an insulating casing having a generally planar bottom wall andsurrounding peripheral side walls extending substantially perpendicularthereto;

(b) at least one relatively stationary contact supported on said bottomwall of said insulating casing;

(c) at least one movable contact movable into and out of engagement withsaid stationary contact;

((1) operating mechanism for moving said movable contact, said operatingmechanism comprising a pair of sheet metallic plates supported ingenerally parallel spaced apart relation in said insulating casing, eachof said plates having a first end portion in engagement with said bottomwall of said insulating casing at a point spaced from said relativelystationary contact, each of said plates having an opposite end portionextending adjacent said stationary contact;

(e) means supporting said opposite end portions of said plates each onone of said side walls of said insulating casing at a point spaced awayfrom said bottom wall in a direction perpendicular to said bottom wall,and

(f) fastening means positively anchoring each of said plates to saidbottom Wall of said insulating casing, said fastening means engagingsaid plates at a point intermediate said end portions.

2. An electric circuit breaker comprising:

(a) an insulating casing having a generally planar bottom wall andsurrounding peripheral side walls extending substantially perpendicularthereto;

(b) at least one relatively stationary contact supported on said bottomwall of said insulating casing;

'(c) at least one movable contact movable along a predetermined path ofmovement between a closed position in which it is in contact with saidstationary contact and an open position in which it is out of con-3 tactwith said stationary contact;

(d) are extinguishing means comprising a series of metallic platessupported in spaced apart relation along said path of movement of saidmovable contact;

(e) operating mechanism for moving said movable contact between saidopen and closed positions, said operating mechanism comprising a pair ofsheet metallic side plates supported in generally parallel spaced apartrelation, each of said side plates having a first end portion inengagement with said bottom wall of said insulating casing at a pointspaced from said relatively stationary contact, said side frames eachhaving an opposite end portion extending at least partially over saidare extinguishing means;

(f) means supporting said opposite end portions each on one of said sidewalls of said insulating casing, and

(g) fastening means positively anchoring each of said side plates tosaid insulating casing, said fastening means engaging said side platesat a point intermediate said end portions.

3. An electric circuit breaker comprising:

(a) an insulating casing having a generally planar bottom wall andsurrounding peripheral side walls extending substantially perpendicularthereto;

(b) at least one relatively stationary contact supported on said bottomwall of said insulating casing;

(c) at least one movable contact movable along a predetermined path ofmovement between a closed position in which it is in contact with saidstationary contact and an open position in which it is out, of contactwith said stationary contact;

(d) are extinguishing means comprising a series of metallic platessupported in spaced apart relation along said path of movement of saidmovable contact;

(e) operating mechanism for moving said movable contact between saidopen and closed circuit positions, said operating mechanism comprising apair of sheet metallic plates supported in generally parallel spacedapart relation, each of said plates having a first end portion inengagement with said bottom wall of said insulating casing at a pointspaced from said relatively stationary contact, said plates each havingan opposite end portion extending at least partially over said areextinguishing means;

(f) means supporting said opposite end portions each on one-of said sidewalls of said insulating casing; (g) fastening means positivelyanchoring each of said plates to said insulating casing, said fasteningmeans engaging said plates at a point intermediate said end portions,and

(h) said plates being dimensioned so that the engagement of said plateswith said casing at said end portion-s prevents said intermediateportions of said side frames from contacting said bottom wall insulatingcasing.

References Cited by the Examiner UNITED STATES PATENTS 2,195,016 3/1940Swingle 200116 2,214,695 9/ 1940 Jennings 200116 2,233,336 2/1941Bentley 200-168 2,454,106 11/1948 Von Hoorn 200-1 16 2,989,604 6/ 1961Wegh 200-116 3,105,132 9/1963 Jencks 200-168 3,171,929 3/1965 Jencks eta1 200116 3,174,024 3/ 1965 Strother et a1 2001 16 BERNARD A. GILHEANY,Primary Examiner.

1. AN ELECTRIC CIRCUIT BREAKER COMPRISING: (A) AN INSULATING CASINGHAVING A GENERALLY PLANAR BOTTOM WALL AND SURROUNDING PERIPHERAL SIDEWALLS EXTENDING SUBSTANTIALLY PERPENDICULAR THERETO; (B) AT LEAST ONERELATIVELY STATIONARY CONTACT SUPPORTED ON SAID BOTTOM WALL OF SAIDINSULATING CASING; (C) AT LEAST ONE MOVABLE CONTACT MOVABLE INTO AND OUTOF ENGAGEMENT WITH SAID STATIONARY CONTACT: (D) OPERATING MECHANISM FORMOVING SAID MOVABLE CONTACT, SAID OPERATING MECHANISM COMPRISING A PAIROF SHEET METALLIC PLATES SUPPORTED IN GENERALLY PARALLEL SPACED APARTRELATION IN SAID INSULATING CASING, EACH OF SAID PLATES HAVING A FIRSTEND PORTION IN ENGAGEMENT WITH SAID BOTTOM WALL OF SAID INSULATINGCASING AT A POINT SPACED FROM SAID RELATIVELY STATIONARY CONTACT, EACHOF SAID PLATES HAVING AN OPPOSITE END PORTION EXTENDING ADJACENT SAIDSTATIONARY CONTACT; (E) MEANS SUPPORTING SAID OPPOSITE END PORTIONS OFSAID PLATES EACH ON ONE OF SAID SIDE WALLS OF SAID INSULATING CASING ATA POINT SPACED AWAY FROM SAID BOTTOM WALL IN A DIRECTION PERPENDICULARTO SAID BOTTOM WALL, AND (F) FASTENING MEANS POSITIVELY ANCHORING EACHOF SAID PLATES TO SAID BOTTOM WALL OF SAID INSULATING CASING, SAIDFASTENING MEANS ENGAGING SAID PLATES AT A POINT INTERMEDIATE SAID ENDPORTIONS.